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304               PRACTICAL ORGANIC CHEMISTRY

I

PREPARATION 94.
Cinnamic Acid.—The reaction, which takes place when an
aldehyde (aliphatic or aromatic) acts on the sodium salt of an
aliphatic acid in presence of the anhydride, is known as
" Perkin's reaction," and has a very wide application. Accord-
ing to the result of Fittig's researches on the properties of the
unsaturated acids described below, the reaction occurs in two
steps, The aldehyde forms first an additive compound with
the acid, the aldehyde carbon attaching itself to the a-carbon
(/.£., next the carboxyl) of the acid. A. saturated hyclroxy-acid is
formed, which is stable, if the a-carbon is attached to only one
atom of hydrogen, as in the case of isobutyric acid,
CII3
CH3X                                              I
Ct;H5CHO +         >CH.COOH = CJI5CH(O1I),CCOO1I.
CII/                                         |
C1I,
If, as in acetic and propionic acids, the group CH2 is present
in the n-position, water is simultaneously split off, and an
unsaturated acid results,
CH3
CuIi3CHO + CHj.CH2.COOH = C6H5CH:C.COOH + H2O.
a-Methylcinnamic acid.
That a-methylcinnamic acid is formed and not phenyliso-
crotonic acid according to the equation,
CGH5CHO + CHo.CHo.COOH = C6Hr,CH:CH.CHo.COOH + IL/J,
Phenylisocrotonic acid.
follows from Fittig's researches, and depends upon the marked
difference exhibited by the two principal groups of unsaturated
acids, viz., the a/3 acids, which have the double link between
the first and second carbon from the carboxyl, and /3y acids, in
which the double link lies between the second and third
carbons. Methylcinnamic acid belongs to the first group,
whereas phenylcrotonic acid belongs to the second group.
It may be noted in passing that this reaction bears a close
resemblance to that studied by Claisen, which occurs in
presence of caustic soda solution between -aldehydes or ketones
on the one hand, and compounds containing the